The present invention relates to an ion inlet for a mass spectrometer. The preferred embodiment relates to apparatus and methods for improving the sampling efficiency of ions in mass spectrometers.
Mass spectrometers often contain different regions or chambers which are at different levels of vacuum. For example, an instrument may have a quadrupole mass filter which resides in a chamber at a pressure of approx. 1×10−5 mbar and which is followed by a collision cell at a pressure of approx. 1×10−3 to 1×10−2 mbar. The collision cell may, in turn, be followed by a Time of Flight mass analyser operating at a pressure of <1×10−6 mbar. These pressures are often achieved by the use of one or more roughing pumps and one or more turbomolecular pumps. Typically, the roughing pump provides the pumping for the source inlet as well as backing the turbomolecular pump(s).
Mass spectrometers can be used with various different source inlet types. The ions are often formed and introduced into the mass spectrometer at atmospheric pressure via a sampling orifice which is located close to the point of ionisation.
The total gas load on the mass spectrometer is defined by the atmospheric pressure orifice. In order to capture the maximum number of ions and therefore maximise the sensitivity of the mass spectrometer, the atmospheric pressure orifice is often located as close as possible to the point of ion formation. In most cases, the atmospheric pressure orifice and the sampling orifice are the same item. These orifices are generally manufactured to be as thin as possible, typically 0.1 mm to 0.5 mm (although thinner and thicker are both known), so as to minimise loss of ion transmission as ions pass through the orifice. The thicker the orifice the more likely it is that some ions will strike the walls of the orifice as they pass through and be lost.
Reducing the size of an orifice (i.e. reducing the diameter of a circular hole or increasing the length of a tube) reduces the gas flow through it, which in turn reduces the quantity of vacuum pumping required to achieve the pressures as described above. However, in the case where the sampling orifice and the atmospheric pressure orifice are the same item, reducing the size of the orifice reduces the volume over which the orifice can sample effectively i.e. ions must pass close to the sampling orifice in order to be drawn in to and through the orifice. Therefore, reducing this orifice size can lead to a significant decrease in the number of ions sampled which reduces the sensitivity of the mass spectrometer. In addition, a smaller diameter orifice is more likely to suffer from a loss of sensitivity over time as contaminants build up on the orifice surface.
It is known that curtain gas can be used to improve the robustness of a sampling orifice. However, the use of a curtain gas often reduces the abundance of ions in the volume in front of the sampling orifice and therefore reduces sensitivity. This is particularly evident as the size of the sampling orifice is reduced.
It is known to have a small atmospheric pressure orifice spatially removed from the ion and gas flow and have a larger sampling orifice in order to improve the long term robustness of the mass spectrometer. However, the total gas flow into the ion sampling orifice is determined by the gas flow into the smaller atmospheric pressure orifice. Due to the larger diameter of the ion sampling orifice, the gas flow velocity into the ion sampling orifice is much lower than at the atmospheric pressure orifice. Therefore, the ability of the atmospheric pressure orifice to capture ions from a high velocity gas flow is reduced.
In addition, in some cases the point of ion formation cannot be located close to the mass spectrometer and so ions must be transferred to the sampling region of the mass spectrometer in order to maximise the ion capture efficiency.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.